There are a number of methods for sensing the presence of speech and other types of signals when received over a communications channel. Such systems are used for a number of purposes including squelch circuit systems, voice operated control systems, etc.
One of the simplest signal presence detection arrangements is based upon the assumption that the signal is normally materially stronger than the noise and thus a simple measurement of amplitude allows one to switch a circuit "on" on the basis of the incoming wave exceeding a specific amplitude level. This circuit works reasonably well for some types of service; however, because communications systems are frequently disturbed by high level noise the setting of the threshold becomes more difficult. If the threshold is set to too sensitive a point, noise will falsely operate the circuit; if the threshold is set to too high a point, weak signals will not be detected by the system.
Other signal sensing arrangements work on the basis of comparing special characteristics of the voice wave, for example, the fact that the low frequency sounds of voice waves are generally significantly stronger than the high frequency sounds. Thus, some systems work on the basis of comparing the high frequency portion of the voice; say above 2,500 Hz, and the level received with the components below 2,500 Hz. If voice is present, the low frequencies' sounds should be noticeably stronger than the high, thus, rather than measuring the amplitude of the signals, a measurement is made of the spectral characteristics of the input wave. This circuit's performance is generally better than a strict amplitude measurement type circuit but there are situations where such a system can be confused. For example, if an interferring tone is received, say at approximately 3 kHz, it can cause the system to operate as though only noise is present and "squelch" the channel even though signal is present. Conversely, a low frequency hum or other type of low frequency interference or noise wave can force this system to falsely indicate the presence of voice.
Another arrangement is disclosed in my reissued U.S. Pat. No. Re. 27,202 wherein the communications channel is split into two or more segments and means are provided for comparing the energy in each segment. The gain for each segment is adjusted so that for the normal type of noise expected, in most cases white noise, the energy in each segment is made equal. The system then makes a determination that noise is present if the weighted energy in the channel is equal and if it is unequal that a signal is present. Such a system has been widely used and provided quite good performance when one type of noise or interference is present.